U.S. Pat. No. 3,597,403 teaches polymerization of certain unsaturated alicyclic compounds with a catalyst comprising (A) at least one compound selected from the group consisting of alkylaluminum dihalides, alkylaluminum sesquihalides and aluminum halides, (B) at least one compound selected from the group consisting of molecular oxygen, chlorine, bromine, iodine and cyanogenhalides, and (C) at least one transition metal compound selected from the group consisting of tungsten and molybdenum carbonyl complex compounds. It is also known that certain hydrocarbon monoolefins may be used to adjust molecular weight of polyalkenamers (U.S. Pat. No. 3,816,382). The very broad teachings of the prior art fail to suggest a polymerization process for production of easy-processing and substantially gel-free norbornenedicyclopentadiene ring-opened copolymers. Such a process and copolymers are desired in order to simplify polymer removal from polymerization vessels.